1. Field of the Invention
The invention relates generally to the field of fracturing fluids used in fracturing subterranean formations during hydrocarbon recovery. More specifically the invention relates to methods for introducing additives in fracturing fluids to supplement or replace traditional biocides.
2. Background Art
Hydraulic fracturing is a formation stimulation technique used to create additional permeability in a producing formation to increase the flow of hydrocarbons toward the wellbore. Typically, during a hydraulic fracturing operation, a high hydraulic pressure is used to fracture the subterranean formation, creating cracks that facilitate the increased flow of hydrocarbons. Often, proppants are used to keep cracks open that are created during the fracturing operation.
Fracturing fluids include a number of components and are most often water-based. These components typically include acids, biocides, breakers, corrosion inhibitors, friction reducers, gels, iron control chemicals, oxygen scavengers, surfactants and scale inhibitors. Fracturing fluids that contain friction reducers to allow higher flow rates are most often termed “slick water” fracturing fluids.
In most traditional hydraulic fracturing operations, much of the fracturing fluid used is recovered. However, in certain formations and operations, the majority of the fracturing fluid that enters the subterranean formation is not initially recovered, but, instead, remains in the formation. This is particularly true for small pore-sized, low permeability formations such as gas-producing shale formations. Some shales may have unfractured permeabilities of 0.01 to 0.00001 millidarcies. Effective porosity of shales may be 0.2% or less. As a result, it may be possible to initially recover only 15% or less of the fracturing fluid, with the rest of the fracturing fluid remaining in situ.
The unrecovered fracturing fluid in the formation may provide a fertile breeding ground for the anaerobic bacteria present in the hydrocarbon-producing formation. Certain types of bacteria, for example, sulfate reducing bacteria (SRB), can be detrimental to both the recovery of the hydrocarbon and the hydrocarbon itself. SRB act to reduce sulfates to sulfides which are detrimental to both the formation itself, as well as to the hydrocarbon recovered. For instance, the SRB may create sludge or slime, which can reduce the porosity of the formation and thereby impede hydrocarbon recovery. SRB may also produce hydrogen sulfide which may sour the hydrocarbon, as well as cause corrosion in metal tubulars and surface equipment.
Typical fracturing fluids include a biocide in order to control of the action of bacteria such as SRB. However, some of these biocides, such as, for instance, glutaraldehye, present environmental issues. Ground water may be contaminated with the biocide, for instance, during fracturing operations, or through spills of fracturing fluids at the surface. Further, more reactive biocides such as oxidizers tend to have a limited life in the formation. This limited life may present a serious problem in low porosity, low permeability formations, where fracturing fluids may remain for a significant period of time due to low mobility.
Other problems exist with traditional fracturing fluids where environmentally sensitivity is an issue. For instance, certain friction reducers and scale inhibitors may be toxic.
What is needed is a method of controlling undesirable bacteria, such as SRB, in small pore-sized, low permeability formations without the use of traditional biocides during hydraulic fracturing operations. Further, what is needed is a fracturing fluid with less toxic components than traditional fracturing fluids.